System for fastening spark plugs of boilers or the like

ABSTRACT

The invention relates to a system for fastening spark plugs ( 4 ) for boilers or the like, comprising a retainer ( 1 ) with an hole housing the ceramic body ( 104 ) of the spark plug ( 4 ) of the spark plug. According to the invention said hole axially projects for a certain length at least on one side of the retainer ( 1 ) such to form a cylindrical housing ( 5 ) for the ceramic body ( 104 ) of the spark plug ( 4 ), a cylindrical tightening ring ( 6, 8 ) being provided having such inner and external dimensions that it can be inserted by force fit between the inner wall of the cylindrical housing ( 5 ) and the external wall of the ceramic body ( 104 ) so to generate a predetermined radial tightening force of the spark plug ( 4 ) and such to obtain a tight seal between one side and the other one of the retainer ( 1 ).

The present invention relates to a fastening system for spark plugs of boilers, burners, hot plates, or the like, comprising a fastening plate or retainer having an hole housing the ceramic body of the spark plug.

Many different kinds of retainers of the type above defined are known. Each one acts for keeping a spark plug in a predetermined position useful for generating ignition sparks for liquid or gaseous fuels. On the contrary another type of spark plug acts for indicating the presence of flame. Said spark plugs generally comprise a coating ceramic member with an elongated shape, generally cylindrical, for heat and electric insulation wherein a metal conducting member is included projecting outside thereof on both ends. To one end the electric conductor is connected, while at the opposite end the spark is generated. The spark plug has to be firmly supported in a determinate position, particularly at a constant in time and predetermined distance from the ground member. Moreover, when condensing boilers have been introduced wherein the furnace is at pressure slightly higher than the atmospheric pressure, it has become essential to fasten the spark plug on the retainer such that a perfect pneumatic pressure tight occurs between the retainer and the ceramic insulator, that is between a side and the opposite side of the retainer. In fact, if the fastening system is not a tight seal one, a vapor draft can come out of the furnace spreading in the environment that sometimes is home environment. Systems known at present do not meet one or both said requirements in a satisfying manner. In fact, systems based on making cuts all around the housing hole in order to push the metal material of the retainer inside the hole against the ceramic wall of the spark plug or based on making permanent deformations in the area about the hole which are pushed against the ceramic wall do not guarantee seal, as well as in many cases the tightening force may be too strong and such to cause cracking or breaking of the ceramic. Moreover it is difficult to guarantee the perfect orthogonally between the retainer and the spark plug due to the reduced contact surface between the retainer and the ceramic body of the spark plug. Retainer type providing at the housing hole for the spark plug a cylindrical member with an axial extension greater than the retainer thickness, and a further cylindrical member as an axially open ring that is fitted on the ceramic body of the spark plug and axially forced inside the cylindrical member in order to obtain an inwards elastic deformation and to tighten the spark plug body, do not guarantee the pressure tight. On the other hand, the drawback of the manufacturing of a shaped ceramic body is a positioning limit of the spark plug on the retainer. The system with threaded spark plug body and fastening screw provides a long mounting and sometimes it is difficult due to overall dimensions of burner components or the location inside furnace cavities in locations difficult to reach or where working areas are small. Moreover it is necessary to provide a great number of components, at least a tightening monkey spanner, and it is necessary to consider the high cost of threading bodies or bushings. Fastening systems by means of clamps of various type lead to a not much strong and safe fastening.

Therefore the present invention aims to overcome all the above drawbacks, and by simple and inexpensive means it allows to manufacture a fastening system of the type disclosed hereinbefore allowing a simple, fast, inexpensive and firm in time fastening of the spark plug on the retainer in a determinate position and with a perfect orthogonal arrangement with respect to the two faces of the retainer, using a very small amount of components. Moreover it is desired that fastening of the spark plug on the retainer is of the pressure tight type, such to allow the use even with condensating boilers.

The invention attains the above aims by means of a fastening system of the type described hereinbefore wherein the hole housing the spark plug axially projects for a certain length at least on one side of the retainer beyond the thickness thereof such to form a cylindrical housing for the ceramic body of the spark plug. It is further provided a cylindrical tightening ring having such inner and external radial dimensions that it can be inserted by force fit between the inner wall of the cylindrical housing of the retainer and the external wall of the ceramic body of the spark plug such to generate a predetermined radial tightening force of the spark plug inside the cylindrical housing of the retainer and such to obtain a tight seal between one side and the other one of the retainer.

The retainer and the ring are made of metal material in order to guarantee a positioning firmness in time of the spark plug. While any metal materials can be used, metal materials having a good thermal resistance are preferred, in the sense of wear thereof due to thermal stress.

Metal materials of the retainer and the ring can be the same or different one with respect to the other. In the case of different metal materials it is advantageous to provide two metal materials whose difference in thermoelectric potential is negligible. That for avoiding electric corrosion situations and eddy current overlapping situations that can influence the proper operation of the spark plug.

The locking fit provides that during the pressure force fit of the ring between the inner edge of the hole housing the spark plug and the external wall of the spark plug, a partial deformation is obtained in the sense of an expansion of the housing hole in the retainer and a partial deformation in the sense of a radial compression of the ring.

The relation between the deformation of the retainer hole in the sense of an expansion thereof and of the ring in the sense of a compression thereof depends on the fact if materials of the retainer and ring are the same or different one with respect to the other or anyway if they have the same or different deformation capacity.

The inside diameter of the hole housing the spark plug has a such size with respect to the external diameter of the spark plug that the difference in diameter is slightly less than the ring thickness.

The ring particularly is a member for compensating and recovering the tightening radial pressure that eventually exceeds a predetermined pressure value for properly tightening the spark plug inside the cylindrical housing of the retainer and such to avoid the breaking or the cracking of the ceramic body.

On the insertion side advantageously the latter has a conical thinning or tapering allowing the entry between inner edge of the hole and external surface of the spark plug in a position for the axial force fit compression in the final position locking the spark plug in the retainer hole.

The fact that the ring and the retainer are made of brass has the advantage to provide the two parts made of a quite ductile metal. However brass has a low resistance to thermal stress and moreover particularly the ring has to be made with forming techniques, that is quite expensive. However in this case the fastening of the spark plug is made in a proper way.

The fact of manufacturing the ring with brass and the retainer with a different material such as steel, stainless steel, or the like allows to manufacture at least the retainer with a less expensive material and that can be processed or molded by shearing. The obtained fastening is also a good one. However the combination of the two different materials leads to a thermoelectric potential that cannot be absolutely ignored and to the arise of deterioration cases due to said difference in potential.

According to a further improvement, there can be provided means for stopping the insertion of the tightening ring inside the cylindrical housing.

In a preferred embodiment, said means can be an annular widening of one of the two circular head edges of the cylindrical tightening ring constituting an abutment annular flange stopping the insertion of the tightening ring inside the cylindrical housing of the retainer.

Moreover, the circular head end of the tightening ring faced to the one having the annular flange may have a beveled or rounded external edge for facilitating the insertion of the tightening ring inside the cylindrical housing of the retainer.

The advantages of the fastening system just disclosed are considerable. The presence of a sort of cylindrical glass housing the spark plug allows the perfect orthogonality between the spark plug and the retainer surface, while the use of a tightening ring made of metal material that can be deformed allows to work in a precise way without exceeding the compression force that the ceramic can support. The maximum axial positioning freedom of the spark plug on the retainer is guaranteed, and the fastening is firm, safe and it avoids casual disengagements. Moreover, the use of a not open cylindrical ring allows to obtain the desired pressure tight between a face and the other face of the retainer.

A second type of the fastening system according to the present invention provides both for the retainer and the ring to use metal materials having a good resistance to thermal stress and generating a negligible difference in thermoelectric potential one with respect to the other at the same time constituting a low cost metal and easy to work particularly by shearing.

Particularly it is advantageous to manufacture both the retainer and the ring with steel or stainless steel. The use of the same material for the retainer and the ring guarantees that a difference in thermoelectric potential is not generated whose micro currents can cause oxidation and corrosion cases in time. If ceramic has through cracks, it is possible to have also micro currents cases generated by retainer-ring and electrode members with respect to the ground, that in the case of electrodes detecting the flame, can be considered as current of present flame signal even when the flame burns out with the valve controlling the gas flow supplying the burner going on in the open condition. In this case unburnt gas can came out from the boiler that can cause dangerous explosions. In the simple case of an ignition electrode, the latter could not properly work.

The present invention overcomes the drawback disclosed above by means of a retainer of the type described hereinbefore wherein even in this case the housing hole axially projects for a certain length at least on one side of the retainer beyond the thickness thereof such to form a cylindrical housing for the ceramic body of the spark plug, but the tightening ring is made of a material, particularly metal, with a low ductility or deformation capacity, particularly it is made of the same material as the retainer, particularly steel, stainless steel or the like.

Steel, and particularly stainless steel have an optimum resistance to thermal stresses and are material relatively inexpensive. Moreover, they allow the manufacture of constructive parts by simply shearing thereof from a continuous sheet, thereby the manufacturing cost of the finished part is generally very low, and manufacturing processes are of high efficiency.

In spite of the low deformation capacity with respect to metals having more ductility, such as brass the ring may be shaped such to have one or more areas for compensating and recovering the tightening radial pressure that eventually exceeds a predetermined pressure value that can be supported by the ceramic.

More specifically, the tightening ring may have one or more openings for the release and recovering of the radial deformation of the ring when the spark plug is tightened inside the cylindrical housing of the retainer.

According to an embodiment, the tightening ring may have at least one, preferably a plurality of notches originating from the two opposite head circular edges of the ring and extending only for a portion of the axial extension of the ring, such to guarantee the pressure tight by the ring itself.

Said notches may have all the same lengthways extension. Particularly said extension may be comprised between about a third and about an half of the axial extension of the ring.

Particularly and advantageously notches have such an axial length to delimit an intermediate continuous annular band of the ring having a very small axial size and such that it can be easily deformed. Generally such intermediate continuous annular band has an axial size of some millimeters and more precisely from 1 to 3 mm.

Said notches can be arranged with alternate positions one with respect to the other, that is each notch can originate from an head circular edge of the tightening ring opposite to the head circular edge from which the immediately next notch originates.

This embodiment allows to compensate the low ductility and the low compressibility of the ring material and at the same time it guarantees the proper seal since it generates no axial continuous through slot.

According to an improvement, the tightening ring at the intermediate annular band may have at least an opening or slot for the release or recover of the radial narrowing occurring in said intermediate annular band when the spark plug is tightened inside the cylindrical housing of the retainer.

Particularly, said slot may be sickle-shaped with the two ends arranged along an axis parallel to the central axis of the tightening ring, and it may extend at a central annular band corresponding to notch end portions arranged at the closed ends thereof.

In a preferred embodiment, the tightening ring may be manufactured as an open one, while maintaining the pressure tight between one side and the other side of the retainer.

In this case, the ring at two faced head edges of a strip that is bent and closed on itself in a cylindrical shape has an hooking recess that narrows as regards its dimension in the axial direction of the ring at the corresponding head edge of the strip forming the ring and which hooking recess cooperates with a hooking extension that has a shape that is complementary to the hooking recess and it has a material portion for the connection to the corresponding head side of the bent strip forming the ring with an axial size smaller than the remaining part of said hooking extension. Said hooking recess and the cooperating hooking extension are complementary means for the reciprocal fastening of faced head sides of the bent strip, that is means for closing the ring.

The recess and extension closing the tightening ring may be provided in substantially intermediate positions of respective straight portions of head sides of the strip forming the ring, with which straight portions said recess and said extension connect in a rounded way.

According to a preferred embodiment, the extension closing the tightening ring may have a root with a narrow shape projecting in the substantially circumferential direction of the ring with a substantially circular shape widened part, while the corresponding recess placed on the opposite end of the tightening ring may have a shape substantially complementary to the one of said extension.

Advantageously, the hooking recess and the corresponding hooking extension of the tightening ring have complementary shapes but the hooking recess has sizes slightly smaller than the hooking extension ones and they can be secured one inside the other by force fit, causing a firm closing of the ring.

Even in this embodiment of the ring, one or both head circular edges of the ring may have an entrance beveling in order to facilitate the insertion by force fit of the ring inside the cylindrical housing.

Due to the arrangements considered in said second embodiment of the invention, if required, it is possible to manufacture the ring with the same material as the retainer guaranteeing both the possibility to recover the tightening radial deformation and the pressure tight between one side and the other side of the retainer. All this is due to the fact that axial notches are not through notches and hooking means of the two faced head sides of the bent strip forming the ring engaging by force fit avoid the leak of gas. Particularly the seal is guaranteed if the ring both is submitted to an inwards compression and an outwards expansion. The ring is easy to manufacture, for example by means of shearing, and subsequent closing on itself. The closing by forcing hooking means on head sides of the strip forming the ring makes it more strong and resistant when it is compressed inside the retainer and it do not require a heat treatment to make it enough strong. As a result a saving in heat treatment cost and in costs due to logistics is obtained since the process is not carried out by the supplier of the part, a vacuum furnace being required.

The constructive manufacturing of the ring by means of a metal material strip that is sheared from a continuous metal material sheet allows to work with a quick and inexpensive manufacturing method and allowing to obtain considerable performances. Moreover the dimensional interference between the hooking recess and the hooking extension is automatically obtained by means of the shearing operation allowing to form the hooking extension of a preceding material strip with the part removed for making the hooking recess of the immediately subsequent material strip. All this is due to the fact that the shearing of the sheet generally causes the cutting of the material for about a third of its thickness, while the remaining two thirds of the material thickness is broken and therefore deformed generating small dimensional differences between hooking recess and hooking extension leading to a reciprocal engagement by force fit.

Therefore a reduction in the manufacturing time due to time necessary for the heat treatment and less off-cuts are obtained. It is also possible to use stainless steel with greater quality as regards the stainlessness degree, since it is not necessary to adapt it for heat treatments. As it is known, steels suitable for heat treatment are not the ones among steels having a greater stainlessness degree. Moreover the material of the ring has no problems inducing thermoelectric phenomena, and it can be used for different insulator diameters. The ring may be manufactured in order to consider different situations and inevitable manufacturing allowances. The assembling is very simple even as regards an industrial automation.

Further characteristics and improvements will form the subject of the dependent claims.

The characteristics of the invention and the advantages derived there from will be more apparent from the following detailed description of the annexed drawings, in which:

FIG. 1 is a perspective view of an spark plug being secured on a retainer according to the present invention by means of a first tightening ring embodiment.

FIG. 2 is an exploded view of the assembly of FIG. 1.

FIG. 3 is a perspective view of the first tightening ring embodiment.

FIG. 4 is a side view of assembly of FIG. 1, in combination with a wall dividing the combustion chamber and the external environment.

FIG. 5 is a top plant view of assembly of FIG. 1.

FIG. 6 is a perspective view of a spark plug being secured on a retainer according to the present invention by means of a second tightening ring embodiment.

FIG. 7 is an exploded view of assembly of FIG. 6.

FIG. 8 is a perspective view of the second tightening ring embodiment.

FIG. 9 is a part section side view, of the second tightening ring embodiment.

FIG. 10 is a plant view of the second tightening ring embodiment in the expanded condition.

FIG. 11 is a perspective view of a variant of the tightening ring according to FIGS. 8 to 10.

FIG. 12 is an axial sectional view of an embodiment of a retainer.

FIG. 13 is a perspective exploded view of the fastening plate, the cylindrical ceramic body of the spark plug and the tightening ring, wherein the ring is made according to the embodiment of FIGS. 1 to 5 while the fastening plate is made according to the embodiment of FIG. 12.

FIG. 14 is a perspective axial section of the fastening plate, the cylindrical ceramic body of the spark plug and the tightening ring according to FIG. 13 in the mounted condition.

FIG. 15 is a perspective view of a further embodiment of the retainer.

Referring to figures, the retainer 1 comprises a substantially flat annular plate-like part connecting to a remaining. part of retainer 1, which has a hole 2 for the passage of a screw fastening the retainer 1 on the boiler. In the preferred shown embodiment, the retainer 1 is completely flat, but it can be fabricated in any shape with respect to the device to which it has to be matched. The retainer 1 further comprises an hole for housing the spark plug 4, which hole has a complementary shape with respect to the external section of the spark plug 4 that generally is fabricated with a cylindrical shape. The spark plug 4 is composed of a cylindrical ceramic body 104 provided with an axial hole 404 inside which there is provided a metal electrode 204 provided with an annular widening 304 stopping the insertion of the electrode 204 inside the hole 404 of the ceramic body 104. With the electrode 204 being fastened inside the hole 404 of the ceramic body 104, said electrode 204 projects to a predetermined extent from both ends thereof. A projecting end has a shape such that it can be connected to an electric conductor for example by means of a Faston type connector or the like, while the opposite end projects from the ceramic body 104 to a greater extent and, in the shown embodiment, it is angularly bent such that, with the spark plug 4 applied on the boiler, it is at a predetermined distance from the ground member for generating the ignition spark. Concerning this, although the present invention is shown in combination with a spark plug 4, it is to be noted that it can be advantageously applied even to fastening of spark plugs detecting the presence of flame, or the like. The cylindrical hole for housing the spark plug 4 axially extends for a certain length on a side of the retainer 1 beyond the thickness thereof towards the ignition end of the spark plug 4 such to realize a cylindrical housing 5 for the ceramic body 104 of the spark plug 4. However, in a different embodiment, it is possible to provide for the cylindrical housing 5 to be faced in the opposite direction or both directions. The inside diameter of the cylindrical housing 5 is greater than the external diameter of the ceramic body 104, a first embodiment of cylindrical tightening ring 6 being provided having such inside and external radial dimensions that it can be inserted by force fit between the inside wall of the cylindrical housing 5 of the retainer 1 and the external wall of the ceramic body 104 of the spark plug 4 such to provide a predetermined radial tightening force of the spark plug 4 inside the cylindrical housing 5 of the retainer 1 and such to have a tight seal between one side and the other side of the retainer 1. The pressure tight between the combustion environment and the external environment can be further guaranteed by the presence of a wall 7 dividing said two environments that is provided inside the boiler. Considering the fact that the retainer 1 is generally entirely made of a material that is substantially not much capable of being deformed, particularly of steel, the tightening ring 6 is made of a material having predetermined deformation and ductility characteristics, particularly metal. According to a preferred embodiment, the tightening ring 6 is made of brass. When the ring 6 is inserted by force fit in the space comprises between the inner wall of the cylindrical housing 5 and the external wall of the ceramic body 104 of the spark plug 4, the ring 6. suffers a slight deformation tightening the ceramic body 104 and firmly secures in position the spark plug 4 on the retainer 1. On the other hand the ductility of brass compensates and recovers the tightening radial pressure that eventually exceeds a predetermined pressure value for proper tightening the spark plug 4 inside the cylindrical housing 5 of the retainer 1 and it avoids the possible breaking of ceramic. Even the retainer suffers a partial deformation relevant to the expansion of the hole housing the spark plug that in this case is less than the deformation of the ring due to the relation between the brass and steel deformation. Due to the fact that the cylindrical housing 5 has a certain axial extension and it is perpendicular to the surface of retainer 1, the perfect orthogonality is guaranteed between the spark plug 4 and the surface of retainer 1, while the presence of the ring 6 with a continuous cylindrical extension guarantees the pressure tight between a side and the other side of the retainer 1. In order to facilitate the insertion of the tightening ring 6 inside the cylindrical housing 5, the circular edge of the cylindrical housing 5 that is coplanar to one of the two faces of the retainer 1 has an entrance flare 105. On the other hand, one of the two circular head edges of the cylindrical tightening ring 6 has an abutment annular flange stopping the insertion of the tightening ring 6 inside the cylindrical housing 5 of the retainer 1. On the contrary the opposite circular head end of the tightening ring 6 has a beveled or rounded edge 206 for facilitating the insertion of the tightening ring 6 inside the cylindrical housing 5 of the retainer 1.

In FIGS. 6 to 10 a second system for fastening the spark plug 4 on the retainer 1 is shown based on a second tightening ring embodiment 8 that in this case is made of the same material as the retainer 1, generally steel, in order to satisfy particular situations wherein the presence of two different metals, that of the ring 6 and that of the retainer 1, can cause the above drawbacks, such as eddy current or the like. Even in this case the spark plug 4 is secured on the retainer 1 introducing by force fit the tightening ring 8 between the inner wall of the cylindrical housing 5 and the external wall of the ceramic body 104 of the spark plug 4. However, as said above, since the tightening ring 8 is made of a material that substantially is not much capable of being deformed or having a greater resistance to deformation, it has a plurality of notches 108 originating from two opposite circular head edges of the ring 8 and extending between about a third and about an half of the axial extension of the ring 8. Said notches 108 are arranged in alternate positions one with respect to the other, that is each notch 108 originates from a circular head edge of the tightening ring 8 opposite to the circular head edge from which the immediately adjacent notch 108 originates and a continuous intermediate annular band is formed there between having a very little size in the axial direction of the ring particularly between 1 and 3 mm. It is also possible that no continuous intermediate annular band is formed, when notches have an extension in the axial direction of the ring that is half of the total axial extension of the ring. During the insertion by force fit of the ring 8 inside the cylindrical housing 5, said notches 108 get narrower in a progressive way starting from the closed ends thereof in the direction of the open ends thereof such to compensate and recover the tightening radial pressure that eventually exceeds a predetermined pressure value for proper tightening the spark plug 4 inside the cylindrical housing 5 of the retainer 1, and such to avoid cracking or even the breaking of the ceramic body 104. The tightening ring 8 is manufactured by shearing and rolling it in a cylindrical shape a strip of metal material and therefore it is substantially an open ring with an opening cut that is rectilinear and parallel to the central axis of the ring 8. Both for allowing the closure of the ring, that is the connection of the two faced head edges of the material strip and for guarantee the pressure tight by the ring 8 between a side and the other side of the retainer 1, an head side on one end of the material strip forming the tightening ring 8 placed at the opening cut has a recess 208, while the head side of the opposite end has an extension 308 intended for the engagement in said recess 208 for closing the tightening ring 8. The recess 208 and the extension 308 closing the tightening ring 8 are provided in positions that are substantially intermediate in the respective straight portions of head sides of ends of the ring 8, and said recess 208 and said extension 308 connect in a rounded way with said straight portions. The extension 308 closing the tightening ring 8 has a root 408 of narrow shape extending in the substantially circumferential direction of the ring 8 with a widened portion of substantially circular shape, while the corresponding recess 208 placed on the opposite end of the tightening ring 8 has a shape that is substantially complementary to the one of said extension 308. However, the size of the extension 308 is slightly greater than that of the hooking recess 208. Due to this arrangement, the extension 308 engages by force fit the recess 208, making a firm mechanical connection of the two faced head ends of the strip forming the ring and generating a seal adhesion of perimetral edges of the hooking recess with the hooking extension.

The arrangement above disclosed regarding the hooking member and the recess with widened head and the rounded shape facilitates the generation of contact areas between the recess edge and the extension edge that make seal areas and therefore avoiding the passage of gas, thus, in the mounting condition, the fastening system according to this embodiment allows to have gas tight conditions need for the use in condensing boilers even if the ring is a substantially open ring.

The manufacturing of the open ring allows in turn the manufacturing thereof by shearing the material strip from a continuous material band. Moreover, considering the effect that the shearing process has on the material, namely that said process incises and cuts the material for about a third of the thickness and for the remaining two thirds of thickness the separation can be compared to a breaking, the deformation generated by the breaking causes an expansion of the extension dimensions and/or a narrowing of the recess dimensions, therefore the relative dimensioning between recess and extension is automatically generated that is necessary to obtain the engagement of said two portions by reciprocal force fit. That allows to obtain the hooking extension and recess without material off-cuts, the removed portion of the material strip forming the hooking recess at the same time forms the hooking extension of a following material strip.

FIG. 11 illustrates a variant embodiment of the tightening ring described with reference to FIGS. 6 to 10. In this variant, the facing ends of the open ring have respectively a protrusion 308′ and a recess 208′ which have a constant axial width. So that in the rolled up form of the metal strip, particularly of the steel strip of FIG. 11, the side edges of the protrusion 308′ and of the recess 208′ which are oriented in the circumferential direction of the ring can slide one against the other. The axial width of the protrusion and of the recess are identical within the limits of high tolerances so that a sealing effect is achieved when the protrusion is fit inside the recess, and in any case at least when the deformation of the ring has occurred after having forced it together with the ceramic body of the spark plug in the retainer bushing. Considering the tightening ring in its normal condition, when it is not mounted in the retainer and pressed between the bushings or cylindrical housing 5 of the retainer 1, the protrusion which is arched correspondingly to the radius of the ring does not completely fit inside the recess but it is in a position with it end at a certain distance of the axial bottom edge of the recess. The depth of the recess and the length of the protrusion are such that between the axial edges of the protrusion and of the recess and the resting facing axial surfaces of the ring on both sides of the protrusion and of the recess a certain distance is maintained helping in allowing to receive the expansion of the material of the ring when it is compressed between the ceramic body and the cylindrical seat 5 of the retainer 1 in order to avoid excessive radial forces on the ceramic body.

In other words the tightening ring according to this variant and in the rolled condition of the metal strip forming the ring, has an axial cut forming two facing ends. The axial cut has a certain width so that the said two ends are at a predetermined distance one form the other. A first end has a central protrusion 308′and the opposite end has a recess 208′ coinciding with the said central protrusion. The central protrusion- is curved essentially with the same radius as the ring and has circumferential side edges extending along a plane perpendicular to the ring axis. The recess has circumferential side edges extending along the same plane perpendicular to the ring axis as the side edges of the said central protrusion 308′.

The axial width of the central protrusion and the axial width of the recess are almost identical such that the protrusion fits in the said recess with the side edges of the said central protrusion coming into contact with the corresponding side edges of the recess. The said recess has an angular extension which corresponds essentially to the width of the axial cut.

The axial width of the protrusion of one end of the tightening ring and the axial width of the recess are such that the side edges of the protrusion and the corresponding side edges of the recess extending in circumferential direction of the ring adheres one to the other in a sealing way, at least after being forced together with the ceramic body of the spark plug (4) in the retainer.

The axial dimension of the protrusion 308′ and of the cooperating recess 208′ are less than the total axial extension of the ring and the said protrusion and the said recess are almost centered relatively to the said total axial extension of the ring. So at both sides of the protrusion 308′ and of the recess 208′ axial end surfaces of the two facing ends of the ring are left.

As it appears clearly form FIG. 11, the ring has also the axial notches 108′ as in the embodiment according to FIGS. 6 to 10.

Relating to the method for building the ring according to the embodiment of FIG. 11, the same method as described for the ring according to FIGS. 6 to 10 can be used.

It has to be noted that during force fitting of the tightening ring inside the retainer 1 together with the ceramic body of the spark plug 4 positioned inside the tightening ring, the radial compression of the tightening ring can be partly compensated by a circumferential sliding of the protrusion 308′ inside the recess 208′. So the expansion of the material of the tightening ring due to the compression is partly compensated by this sliding and the radial compression forces exerted on the ceramic body can be maintained below the threshold which ensures that no damage is caused to the ceramic body of the spark plug 4 but a tight securing of the said spark plug is obtained which also ensures a sealing against passage of gas.

FIG. 12 illustrate an embodiment of the retainer 1. According to this embodiment the cylindrical seat 5 for the ring and for the ceramic body of the spark plug 4 has a first entrance flare 105 at the circular edge of the cylindrical housing 5 that is coplanar to one of the two faces of the retainer 1 as described with respect to the embodiment of FIG. 1 to 5 which is followed by a cylindrical part extending for the width of the fastening plate 101 of the retainer 1. On the opposite side of the plate 101 a frustoconical part 205 reduces further the internal diameter of the cylindrical seat to the end diameter of the cylindrical seat formed by a cylindrical bushing 305. As it appears clearly the thickness of the fastening plate is bigger than the thickness of the walls of the cylindrical bushing or extension 5 forming the seat for the ceramic body of the park plug 4 and of the tightening ring 8. The above internal shape of the seat allows to maintain the diameter of the hole passing the fastening plate 101 with a diameter slightly larger. This allows to avoid that high radial stresses are exerted on the ceramic body of the spark plug at the crossing of the fastening plate 101 which has a high radial resistance to deformation due to its increased thickness.

The bushing itself, or at least the cylindrical seat 5 has a reduced thickness of the walls, particularly for the part having the final correct diameter for press fitting the tightening ring and the ceramic body of the spark plug 4. This allows that radial forces can be at least partially absorbed by a deformation not only of the tightening ring but also of the cylindrical seat 5, i.e. of the terminal cylindrical extension 305.

An increased thickness of the fastening plate 101 of the retainer is very helpful since during securing of the plate by means of screws passing through holes 201 in the fastening plate 101 the fastening plate could arcuate or bend or be submitted to other deformations when the screws are tightened. This is particularly relevant not only for the correct positioning of the spark plug 4 relatively for example to a burner, but also in connection with gas tight combustion chambers, since a deformation of the fastening plate could lead to leakage.

On the other hand the retainer can be built in a simple manner by drawing or pressing since the material which has to be eliminated for forming the hole in the fastening plate for the spark plug is sufficient to form the seat 5 with a reduced wall thickness.

FIGS. 13 and 14 illustrates the ceramic body of the spark plug 4 and the tightening ring 6 according to the embodiment of FIGS. 1 to 5 in combination with a retainer according to FIG. 12 and respectively in the non mounted condition and in the condition in which the ceramic body of the spark plug 4 with the tightening ring 6 are forced inside the retainer 1. This in order to make clear that the retainer according to FIG. 12 can be used in combination with every one of the embodiments of the rings 6 or 8 described above.

Relating to the thickness of the fastening plate and of the walls of the cylindrical seat 5 good results has been obtained with a fastening plate having a thickness of at least 3 mm, while the walls of the cylindrical seat 5 has a thickness of less than 2 mm.

Particularly the fastening plate has a thickness from about 3 mm to about 5 mm, while the walls of the cylindrical seat 5 has a thickness of about 1 mm to 2 mm.

Analogous results in stiffening the fastening plate relatively to deformation at the fastening of the said plate can be obtained with a retainer according to the embodiments of FIGS. 1 to 10 by providing the fastening plate of the retainer with stiffening ribs.

FIG. 15 illustrates such a stiffened fastening plate 101 which in this embodiment is provided with a peripheral rib 301. The thickness of the plate 101 is less than 3 mm according to the plate 101 of the embodiment of FIGS. 1 to 10 and the stiffening rib is obtained by deformation of the peripheral edge of the plate 101 by means of stamping or drawing.

A further feature which appears in the embodiment of FIG. 15 and which can be provided for each of the different embodiments of the fastening plate 101 of the retainer 1 consist in a mean for connecting electrically and mechanically a conductor, particularly a cable for generating an electrical connection to earth potential.

The mean for connecting electrically and mechanically a conductor consist in a shaped protrusion forming the male of female part of a electrical and/or mechanical connector which other respectively female or male part is fastened to the end of the cable.

In its simplest form the mean for connecting electrically and mechanically a conductor consist in a shaped protrusion, such as a metallic tongue 401 which is dimensioned and shaped in such a way as to cooperate with the female part of a so called FASTOM® connector.

Particularly the tongue is made in one piece with the retainer and it is obtained together with the other features of the retainer in a one shot operation of stamping or drawing from a metallic sheet.

The retainer can be made of any kind of metal, particularly of brass, copper, copper-nickel alloy, steel.

Relating to the embodiment of the retainer according to FIGS. 12 to 14, particularly FIG. 14 shows that in contrast to the embodiment according to FIGS. 1 to 5, the hole in the fastening plate 101 for housing the ceramic body 104 of the spark plug 4 can have advantageously such a radius that the flange 106 of the tightening ring 6 passes through this hole, namely the said hole has a larger radius as the external radius of the flange 106. In this case the flange 106 abuts against the inner surface of the reduction of the frustoconical part 205 on the other side of the fastening plate 101, which has a larger radius and a shorter radius being respectively bigger and smaller than the external radius of the flange 106 of the tightening ring 6. Thus the said flange 106 and the corresponding end of the ring 6 are completely embedded within the thickness of the fastening plate 101. Thus from FIG. 14 it appears clearly that the trough hole in the fastening plate 101 for the ceramic body of the spark plug 4 has a greater diameter than the said ceramic body together with the tightening ring on it, while the cylindrical seat 5; 305 with the reduced diameter for force fitting the said ceramic body and the tightening ring is provided on one side of the fastening plate 101 outside the thickness of the fastening plate and is connected to the said hole of the fastening plate 101 with a frustoconical part 205.

The cylindrical seat 5; 305 with the reduced diameter for force fitting the said ceramic body and the tightening ring eventually together with the said frustoconical part 205 have an axial extension corresponding at least to the axial extension of the tightening ring.

Obviously, the invention is not limited to the embodiments described and illustrated herein but may be greatly varied, especially as regards construction, without departing from the guiding principle disclosed above and claimed below. 

1. System for fastening spark plugs (4) for boilers or the like, comprising a retainer or a fastening plate (1) with an hole housing the ceramic body (104) of the spark plug (4), characterized in that the housing hole axially projects for a certain length at least on one side of the retainer (1) beyond the thickness thereof such to form a cylindrical housing (5) for the ceramic body (104) of the spark plug (4), a cylindrical tightening ring (6, 8) being provided having such inner and external dimensions that it can be inserted by force fit between the inner wall of the cylindrical housing (5) of the retainer (1) and the external wall of the ceramic body (104) of the spark plug (4) so to generate a predetermined radial tightening force of the spark plug (4) inside the cylindrical housing (5) of the retainer (1) and such to obtain a tight seal between one side and the other one of the retainer (1).
 2. System according to claim 1, characterized in that the retainer and the ring are made of different metal materials having a different resistance to deformation.
 3. System according to claim 2, characterized in that the retainer (1) is made of a metal material having a greater resistance to deformation with respect to the ring (2), the tightening ring (6, 8) being the member for compensating and recovering the tightening radial pressure that eventually exceeds a predetermined pressure value for the proper tightening of the spark plug (4) inside the cylindrical housing (5) of the retainer (1).
 4. System according to claims 1 and 2, characterized in that the tightening ring (6) is made of brass.
 5. System according to claim 1 or 2 characterized in that the tightening ring is made of a copper-nickel alloy.
 6. System according to claim 1, characterized in that both retainer (1) and the ring (2) are made of brass.
 7. System according to claim 1, characterized in that the retainer (1) and the ring (2) are made of different metal materials and have a negligible difference in the thermoelectric potential.
 8. System according to one or more of the preceding claims, characterized in that the circular edge of the cylindrical housing (5) for the spark plug (4) that is coplanar with one of the two faces of the retainer (1) has an entrance flare (105) in order to facilitate the insertion of the tightening ring (6) inside said cylindrical housing (5).
 9. System according to one or more of the preceding claims, characterized in that stop means (106) for the insertion of the tightening ring inside the cylindrical housing (5) are provided.
 10. System according to one or more of the preceding claims, characterized in that said means are composed of an annular widening of one of the two circular head edges of the cylindrical tightening ring (6) that forms an abutment stop annular flange (106) for the insertion of the tightening ring (6) inside the cylindrical housing (5) of the retainer (1).
 11. System according to one or more of the preceding claims, characterized in that the circular end of the tightening ring (6) opposite to the one having the annular flange (106) has a beveled or rounded or tapered edge (206) in order to facilitate the insertion of the tightening ring (6) inside the cylindrical housing (5) of the retainer (1).
 12. System according to one or more of the preceding claims, characterized in that the tightening ring (6) is made of a material, particularly metal, having a high resistance to deformation capacity, particularly it is made of the same material as the retainer (1), and it is shaped such to have one or more areas (108, 9) for compensating and recover the radial tightening pressure that eventually exceeds a predetermined pressure value for the proper tightening of the spark plug (4) inside the cylindrical housing (5) of the retainer (1).
 13. System according to one or more of the preceding claims, characterized in that the tightening ring (8) has one or more openings (108, 9) for the release and recover of the radial deformation of the ring (8) when the spark plug (4) is tightened inside the cylindrical housing (5) of the retainer (1).
 14. System according to one or more of the preceding claims, characterized in that the tightening ring (8) has at least one, preferably a plurality of notches (108) originating from the two opposite circular head edges of the ring (8) and extend only for a portion of the axial extension of the ring (8).
 15. System according to one or more of the preceding claims, characterized in that said notches (108) all have the same lengthways extension, particularly said extension is comprised between about a third and about an half of the axial extension of the ring (8).
 16. System according to one or more of the preceding claims, characterized in that said notches (108) are arranged in alternate positions one with respect to the other, that is each notch (108) originates from a head circular edge of the tightening ring (8) opposite to the head circular edge from which the immediately next notch (108) originates.
 17. System according to claim 16, characterized in that notches (108) form there between an annular continuous intermediate band, having a small thickness with respect to the total axial extension of the ring (8).
 18. System according to claim 17, characterized in that said annular continuous intermediate band has an axial dimension comprised between 1 and 3 mm.
 19. System according to one or more of the preceding claims, characterized in that the tightening ring (8) at its intermediate annular band has at least an opening (9) or slot for the release and recover of the radial narrowing occurring in said intermediate annular band when the spark plug (4) is tightened inside the cylindrical housing (5) of the retainer (1).
 20. System according to one or more of the preceding claims, characterized in that the tightening ring (8) is of the open type.
 21. System according to claim 20, characterized in that the tightening ring (8) has reciprocal fastening means by forcing the two faced ends at the opening cut.
 22. System according to claim 21, characterized in that one of the faced ends of the tightening ring (8) placed at the opening cut has at least a recess (208), while the other opposite end has at least an extension (308) intended to engage with said at least one recess (208) for closing the tightening ring (8) on itself the dimensions of the recess and extension (208, 308) being such to engage one inside the other by force fit generating reciprocal contact perimetral surfaces forming a tight closing of the open notch of the ring. (8) itself.
 23. System according to claim 22, characterized in that the hooking recess (208) extension (308) of the tightening ring (8) are provided in substantially intermediate positions of the respective straight end portions of the ring (8), with which straight portions said recess and said extension connect in a rounded way.
 24. System according to one or more of the preceding claims 21 to 23, characterized in that the hooking extension (308) of the tightening ring (8) has a narrowed shape root (408) projecting in substantially circumferential direction of the ring (8) with a substantially circular shape widened part, while the corresponding hooking recess (208) placed on the opposite end of the tightening ring (8) has a shape that is substantially complementary to the one of said extension (308).
 25. System according to claim 20, characterized in that the tightening ring (8) has an axial cut forming two facing ends, the axial cut having a certain width so that the said two ends are at a predetermined distance one form the other, a first end having a central protrusion (308′) and the opposite end having a recess (208′) coinciding with the said central protrusion, the central protrusion (308′) being curved essentially the same radius as the ring and having circumferential side edges extending along a plane perpendicular to the ring axis and the recess (208′) having circumferential side edges extending along the same plane perpendicular to the ring axis as the side edges of the said central protrusion (308′) the axial width of the central protrusion and the axial width of the recess (208′) being identical such that the protrusion (308′) fits in the said recess (208′) with the side edges of the said central protrusion (308′) coming into contact with the corresponding side edges of the recess (208′), the said recess (208′) having an angular extension which corresponds essentially to the width of the axial cut.
 26. A system according to claim 25, characterized in that the axial width of the protrusion (308′) of one end of the tightening ring and the axial width of the recess (208′) are such that the side edges of the protrusion (308′) and the corresponding side edges of the recess (208′) extending in circumferential direction of the ring adheres one to the other in a sealing way, at least after being forced together with the ceramic body of the spark plug (4) in the retainer (1).
 27. A system according to one or more of the preceding claims, characterized in that the retainer (1) is formed by a fastening plate (101) having a hole for housing the ceramic body of a spark plug (4), the said whole being coaxial with a cylindrical extension (5) at one side of the said fastening plate (101).
 28. A system according to claim 27 characterized in that the fastening plate (101) has a thickness greater than the thickness of the cylindrical extension (5).
 29. A system according to claim 28, characterized in that the fastening plate has (101) a thickness of at least 3 mm, while the cylindrical extension (5) has a thickness of less than 2 mm.
 30. A system according to claim 29, characterized in that the fastening plate (101) has a thickness from about 3 mm to about 5 mm, while the cylindrical extension (5) has a thickness of about 1 mm to 2 mm.
 31. A system according to one or more of the preceding claims 28 to 30, characterized in that the trough hole in the fastening plate (101) for the ceramic body of the spark plug (4) has a greater diameter than the said ceramic body together with the tightening ring on it, while the cylindrical seat (5; 305) with the reduced diameter for force fitting the said ceramic body and the tightening ring is provided on one side of the fastening plate (101) outside the thickness of the fastening plate and is connected to the said hole of the fastening plate (101) with a frustoconical part (205).
 32. A system according to claim 31, characterized in that the cylindrical seat (5; 305) with the reduced diameter for force fitting the said ceramic body and the tightening ring has an axial extension corresponding at least to the axial extension of the tightening ring.
 31. A system according to one or more of the preceding claims, characterized in that the fastening plate (1) has a thickness less than 3 mm and has stiffening ribs.
 32. A system according to claim 31, characterized in that the stiffening ribs are formed by pressing or drawing.
 33. A system according to one or more of the preceding claims characterised in that the retainer has means for connecting electrically and mechanically an electric conductor.
 34. A system according to claim 33, characterised in that the said means for connecting electrically and mechanically an electric conductor consist in a male or female part fo an electric and or mechanical connector.
 35. A system according to claim 34, characterised in that the said means for connecting electrically and mechanically an electric conductor consist in the metallic tounge (401) forming the male part of a so called FASTOM® connector.
 36. A system according to one or more of the preceding claims 33 to 35, characterised in that the said means for connecting electrically and mechanically an electric conductor are made in one piece with the fastening plate (101) of the retainer (1).
 37. A system according to claims 35 or 36, characterised in that the said means for connecting electrically and mechanically an electric conductor are obtained in a one shot shaping and cutting operation of the retainer by means of drawing or stamping a metallic sheet.
 38. Method for manufacturing a locking ring by force fit for a system according to one or more of the preceding claims 20 to 26, characterized in that said ring is obtained by shearing a material strip from a metal material band, the hooking extension or the protrusion in a first material strip for a first ring being obtained from the material removed for manufacturing the hooking recess of the immediately next material strip, while said two material strip are rolled in a cylindrical way and then the ends of said cylindrically rolled strips are connected one with the other by force fit engagement of the hooking extension to the hooking recess or of the protrusion in the recess, the differences in dimension between the hooking recess and the hooking extension and of the protrusion in the recess necessary to the dimensional interference for the reciprocal, sealing engagement by force fit of the hooking extension and the hooking recess or of the protrusion in the recess are automatically obtained with the shearing that is carried out in such a way to cut the material band only for a thickness part and to break the material band for the remaining part of the band thickness consequently with the narrowing of the hooking recess or of the recess and the expansion of the hooking extension or of the protrusion due to the material stretching as regards the thickness part of the material band that is separated by breaking it. 